The invention relates to a method of optimizing the production capacity and the flexibility of the product properties when manufacturing chipboards, fibre boards and the like boards by a continuous process, where a thermosetting binder is applied onto the raw material in form of biomass particles, such as chips, fibres and the like, said raw material being spread on a preforming band to form an endless mat, where said mat is preferably pre-compressed in a continuously operating prepress and finally pressed in a continuously operating hot press in such a manner that said mat is compressed into the desired thickness of the finished plate and the thermosetting binder is hardened.
Above all, the hot press is essential to the production capacity of an apparatus and for the properties of the product, said hot press having two basic functions viz. to compress a mat of biomass particles glued to the desired thickness of the plate and to heat said mat to a temperature causing a hardening, i.e. a polymerisation/condensation of the binder.
For this purpose, two types of hot presses are used, viz. conventional step presses pressing a section of the mat per pressing cycle and continuously operating through-type presses advancing an endless mat by means of steel bands through a wedge-shaped slot between two pressing planes with the result that said mat is gradually compressed and full-hardened by means of heat from said pressing planes and said steel bands. These modern presses become more and more important and they are expected to dominate the market. The invention is in particular directed towards a use in connection with this type of press.
Below reference is only made to a continuous press, and the capacity of said press depends on
the capacity of the press for transferring heat from the pressing planes to the steel bands. In this connection, the shape of the roller or slide systems between the pressing planes and the steel bands is of decisive importance, and
the transfer of heat from the steel band to the mat of wood particles and through said mat which is to be heated to approximately 105 to 110xc2x0 C. in the middle in order to harden the binder.
In practice, the heat transfer in the mat turns out to be the limiting factor. The thermal conductivity in the mat is very poor, and accordingly attempts have been made at optimizing the so-called xe2x80x9cDampfstoss-Effektxe2x80x9d, which is a German technical term meaning that the moisture in the surface of the mat evaporates and moves towards the centre of the mat where the steam condenses and releases its evaporation heat.
FIG. 4 shows an example of the temperature course at four different depths of the mat versus the time and consequently the position of the measuring location above the pressing length. The curve segments with a steep temperature gradient represent the xe2x80x9cDampfstoss-Effektxe2x80x9d in the layer in question. The flat temperature gradients represent the heat conducting phase taking over when steam is no longer supplied from the outside.
It appears that the heat conducting phase requires most time and restricts the advancing speed and consequently the capacity of the press.
Thus the xe2x80x9cDampfstoss-Effektxe2x80x9d is the ideal mechanism for transferring heat.
It is, however, subject to limitations because a high steam pressure in the middle layer may cause steam burstings in said middle layer when the plate is leaving the press. The more water/steam that is supplied for heating the mat, the more time the plate must remain under a slight pressure in the press so that the steam can finally condense or escape from the middle layer.
Thus an optimizing of the capacity of the press by means of water/steam dosing presents a compromise between two counter-acting effects.
The conventional method of pressing chipboards or fibre mats in a continuous hot press has, however, not only a limiting effect on the capacity of the press, but also a negative effect on the properties of the product.
The latter situation has been illustrated in the following example showing a conventional pressing of a fibre mat into an MDF plate, cf. FIG. 6A.
A precompressed 80 mm thick mat of glued wood fibres with a moisture content of 9 to 10% corresponding to a 16 mm thick MDF plate is introduced in a continuous press and subjected to a compressing in the first section of the press by means of a very high pressure, usually of the magnitude of 40 to 50 kp/cm2, into a thickness usually being 5 to 10% smaller than the final thickness of the plate, cf. FIG. 6A-2. FIG. 6A-2 shows the distance of the pressing planes, i.e. the thickness of the mat, over the length of the press, and FIG. 6A-1 shows the specific pressure in the mat over the length of the press.
The high pressure in the first phase and the heating from the press bands (txcx9c200xc2x0 C. or more) result in a plastifying and compression of the fibres in the outermost layer of the mat into a density often in the range of 1000 to 1100 kg/m3 for standard MDF-plates.
The pressure is then reduced in the second phase to for instance 1 to 3 kp/cm2 so as to improve the permeability of the middle layer to the steam penetrating from the heated cover layer. As a result the thickness of the mat increases to approximately 25 mm in the illustrated example.
After the heating of the mat to approximately 100xc2x0 C., the distance of the pressing planes is adjusted to the final thickness of the plate with the effect that the pressure is increased to for instance 5 to 10 kp/cm2 so as finally to decrease towards 0 at the termination of the third phase, viz. the calibration phase.
The described method is a method known especially within the MDF industry and it is suited for achieving specific density profiles, cf. FIG. 5. It is, however, encumbered with a few essential draw-backs which can be avoided by the use of the invention:
The high pressure in the first phase presents very high mechanical requirements to the press, and it involves a risk of band and rollers being damaged when the mat contains foreign bodies, such as compact fibre lumps, glue lumps and the like being undetectable by means of a metal detector.
The very low pressure in the second phase is necessary due to the penetration of steam into the middle layer and the heating of said middle layer, but it implies that the glue full-hardens partially without the particles having sufficient mutual contact.
The terminating compression during the calibration in phase 3 is even worse for the process because the glue bridges established under the low pressure in phase 2 are broken under the higher pressure in phase 3.
All things considered, this method is solely intended for achieving a specific density profile, but it is not suited for achieving an optimum utilization of the binder. Thus the transverse tensile strength of the plate can vary a great deal, and the damage in the middle layer is not always associated with the lowest density, cf. FIG. 7.
Various suggestions have been made:
A drying of the wood material to a low moisture percentage, such as 5 to 6% followed by a spraying of water on the mat immediately before the press. The latter is in principle an efficient method because the potential amount of steam for the heat transfer is increased without increasing the total amount of moisture and consequently the risk of steam burstings. It is, however, difficult to control the procedure, and in addition it is not possible immediately before the press to apply water onto the bottom side of the mat. The result can be asymmetrical cross sections of the plates and curved plates.
A preheating of the mat by means of high-frequency waves to 50 to 60xc2x0 C. or more in such a manner that the necessity for a heating in the press is reduced to a level which can be established by means of a moderate xe2x80x9cDampfstoss-Effektxe2x80x9d. The process is difficult to control because even insignificant moisture variations in the mat result in a heterogeneous heating, the dielectric constant of water being approximately 80 times higher than the one for wood. In addition, a heating of the middle layer involves a plastifying which is not desired because the middle layer must be able to offer resistance at the compressing and hardening of the surface of the mat during the first phase of the pressing.
A preheating and a setting of the optimum moisture content in the mat have furthermore been tested by means of
superheated steam of a temperature of 110 to 140xc2x0 C.,
conditioned hot air carried through the mat before the hot press and of a dew point temperature corresponding to the desired moisture content.
The patent literature discloses several methods based on the above principles. These methods are characterised by trying to obtain a flow through the mat and consequently a uniform temperature and a homogeneous moisture content in the entire cross section of the mat.
The above methods are not advantageous because of the undesired plastifying of the middle layer and the not-optimum xe2x80x9cDampfstoss-Effektxe2x80x9d, where the moisture content and the temperature are also increased in the middle layer of the mat, and accordingly it is the object of the invention to obtain a specific and controllable gradient of the moisture content and the temperature in the mat immediately before the continuously operating press.
This object is according to the invention obtained by subjecting the mat immediately before the introduction into the hot press to a pretreatment with steam, whereby the length being subjected to the steam treatment depends on the measured density profile in such a manner that a gradient of the moisture content/temperature is obtained across the thickness of the mat which is optimal with respect to the plastifying degree for a desired product quality and a predetermined pressing process. As a result, the capacity of the apparatus can be increased at the same time as the energy consumption is reduced. Furthermore, the dimensions of the press can be reduced.
Moreover, the mat may according to the invention have a temperature of preferably below 40xc2x0 C. before the pretreatment.
Furthermore, the mat may according to the invention have a moisture content of preferably less than 5% relative to the dry weight of said mat before the pretreatment.
The pretreatment can advantageously be carried out with saturated water steam at a temperature of 100 to 115xc2x0 C., preferably 102 to 110xc2x0 C., especially in the range 104 to 108xc2x0 C.
Moreover, the pretreatment may according to the invention be carried out at a steam pressure of 0.1 to 0.5 bar overpressure, preferably 0.2 to 0.4 bar overpressure.
The introduction of steam may advantageously be controlled such that the gradient of temperature and the moisture content are adjusted to the subsequent hot pressing parameters and the plastifying and compressing of the mat in order to achieve a predetermined density profile of the finished plate. The pretreatment is controlled such that steam burstings in the finished plate in the press outlet are avoided partly by way of an optimizing of the moisture profile in the mat and partly by way of keeping the total moisture content in the mat at less than 10%, preferably less than 8% of dry weight of the mat.